Conventionally, a friction material to be used for brakes and the like has been produced by using a fiber base material, a friction modifier, a filler and a binder, blending them, and performing a production process including steps such as preforming, thermoforming and finishing. In the friction material to be used for brakes and the like, there have been used the fiber base material such as organic fiber such as aramid fiber, inorganic fiber such as glass fiber, or metal fiber such as copper fiber, the organic/inorganic friction modifier such as rubber dust, cashew dust, metal particles, ceramic particles or graphite, the filler such as calcium carbonate or barium sulfate, and the binder such as a phenol resin.
However, for example, in the case of a disc pad, there is a possibility to cause the partial generation of rust on a rotor side thereof in contact with the friction material during parking because of the interposition of water. The rotor and the disc pad are seized to each other by the rust which is partially generated on the rotor, and seizure force due to corrosion causes the occurrence of noise at the time of starting.
A cause for the generation of the rust is that the rotor is placed under an acidic environment, and from the viewpoint of materials of the friction material, low-pH raw materials are considered to be contained. As a method for eliminating the influence of such corrosion accelerating substances from the friction material, calcium hydroxide or the like is sometimes blended for the purpose of pH control.
For example, Patent Document 1 describes a friction material obtained by forming and curing a friction material composition mainly including a fiber base material, a binder and a filler, in which an alkaline raw material having pH of 11.0 or more, such as hydrated lime, is added to control the pH of the whole friction material, the amount of sulfate ions eluted from the friction material, which is measured according to JIS K 0127, is 0.2 mg/g or less, and the pH of the friction material is 10.0 or more and less than 13.0.
Further, Patent Document 2 discloses a friction material to be used in combination with a sliding member made of an aluminum-based metal composite material, in which the hydrogen ion concentration of the friction material is from pH 6 to 10, and the electric resistance value thereof is 300 kΩ or more. However, it is mentioned that in order to control the pH of the friction material to the range as described above, as an inorganic filler, an inorganic filler having pH of 12 or more, for example, calcium hydroxide, is preferably blended in an amount of 1% by volume or less.
Furthermore, Patent Document 3 describes that in a friction material for a small-sized brake lining (small-sized BL), both of a Morning Effect and the generation of ‘Groan’ noise after standing under a high-humidity environment after high-temperature thermal history are prevented at the same time by a friction material containing at least from 7% by volume to 30% by volume of hydrated lime as a filler and from 2.0% by volume to 3.5% by volume of aluminum particles.
Patent Document 4 describes an efficient production method of a friction material including a mixing step of stirring raw materials of the friction material while suppressing a rapid increase in viscosity during mixing the raw materials of the friction material containing a phenol resin, calcium hydroxide and water, and a forming step of forming a mixed powder obtained in the mixing step into a desired shape.
On the other hand, in the case of a non-asbestos friction material, in order to improve fade resistance, metal having a high thermal conductivity, particularly copper fibers or copper particles are added in some cases.
A fade phenomenon is caused by decomposition gas generated when organic substances contained in the friction material are decomposed at high temperature under high load, and heat radiation properties of the friction material itself are improved by adding copper having a high thermal conductivity to the friction material, thus being able to suppress the generation of the decomposition gas.
In recent years, however, from concerns about environmental pollution such as river or sea pollution and adverse effects on human body, it has been desired to develop a friction material containing no heavy metal such as copper.